1. Field
This invention relates to a method for anaerobically fermenting biologically degradable organic material, whereby this material is mixed with a quantity of already fermented material as an inoculum for the active anaerobic fermentation, and whereby this mixture is introduced at the top into a fermentation chamber in which a fermenting mass is situated, which moves from an inlet situated at the top towards an outlet situated at the bottom.
B. Related Art
By organic material, here in particular the organic fraction of domestic waste is intended, and of similar industrial waste and other organic fractions, such as, for example, slurry from water purification installations, slurry from the paper industry, or other kinds of organic slurry.
Methods for anaerobically fermenting organic waste can be divided into wet and dry manners of fermentation.
With wet fermentation methods, fresh or recycled water is added to the solid organic waste or slurry in order to form a highly liquid mash or slurry which can be pumped easily and which can easily be mixed in a fermentation tank. Such mash with 6 to 10% of dry matter, depending on the viscosity of the starting material to be treated, is pumped into a fermentation tank and is transformed into biogas, in mixed fermentation systems, in one or two phases and with mesophilic or thermophilic temperatures.
Due to the high water content of the material in the fermentation tank, the density in this tank is relatively uniform, and the produced biogas can easily escape from the mixed mash.
With a dry fermentation, on the contrary, the quantity of water which is added is limited, such that a relatively solid biologically degradable waste is pumped or pushed into a “dry” fermentation tank by means of special pumps. For organic fractions originating from domestic waste, the dry matter content of the material fed into the tank is between 15 and 45%. For more viscous materials, this may be up to 10 to 15%.
In most cases, with dry fermentation the material is pumped at the bottom into the standing tank, whereby gas is injected in order to obtain a mixing and a horizontal movement of the material. The tank may also be installed horizontally and may be provided with a mixing device which realizes the horizontal movement.
Such method of introducing the material at the bottom of a standing tank is described in WO 86/05200. Biogas is brought into the fermentation tank at different locations, through the underside, such that this underside is divided into sectors. The biogas provides for a mixing of the contents of the sectors and a movement of the fermenting mass from one sector to the other and finally towards the outlet, also situated at the bottom.
EP-A-0.476.217 describes a method, whereby a horizontal fermentation tank with therein a mixing device is used. After having been mixed with a portion of fermented material, the biodegradable material is pushed, through a feeding tube realized as a heat exchanger, into the tank and in this tank is mixed by a blender, as a result of which the fermenting mass flows horizontally through the tank, from one extremity to the other, where it is discharged at the bottom.
According to EP-A-0.205.721 and EP-A-0.577.209, the fermentation is performed in a vertical fermentation tank, without a mixing device therein. The biodegradable material is mixed with fermented material as an inoculum and is pumped into the tank at the top.
In the tank, the fermenting mass sinks, as fermenting material is discharged at the bottom. This latter is performed by means of a sliding grid moving to and fro above the flat bottom of the tank, which grid pushes this fermented material into a screw conveyor device situated therebelow. This device pushes the fermented material into another screw functioning as a lock by forming a stopper of fermented material at its conical outlet.
In the method according to EP-A-0.577.209, moreover, due to the design of the device, a fraction division into a liquid and a solid fraction is obtained, such that the content of solid matter in the fermentation tank can be maintained and that the dry matter content of the fed material, which consists of a mixture of inoculum and fresh material, is situated between 15 and 40%.
In these fermentation tanks without mixer, it is typical that the content thereof rises and expands as a result of the produced biogas. This biogas can not bubble immediately upward as in a wet tank and can not easily escape, due to the high viscosity of the material in which it is created and through which it must weave its way upward.
In practice, the material will expand as a consequence thereof, and its density will be reduced by about 10 to 40%. The average density depends on various parameters, such as the degree of biogas production, the feeding frequency, the kind of structure of the added material, and the height of the feeding tank. Depending on the feeding, the density may vary from 0.7 to 1.2 kg/l.
It was noted that this feeding matter, being a mixture of fresh biodegradable waste and fermented material or residue, has a density which mostly is higher than 1.0 kg/l, on account of the fact that the prevailing biogas during mixing of the fermented material and the fresh material can escape from the fermented material.
When this feeding matter with fresh material and inoculum, which, due to the degassification, has obtained a higher density, is brought at the top into the fermentation tank in which the material has a lower density, then, depending on the difference in density, the dimensions and the diameter of the fermentation tank and the extraction system thereof, too fast a sinking of this feeding matter through the mass in the tank can be noted, such that this feeding matter arrives faster at the bottom than the remainder of the fermenting mass and, thus, is discharged from the tank without having undergone an optimum fermentation.